System and method for distortion limiting

ABSTRACT

A system is provided for limiting distortion of an audio speaker. The system includes a first lowpass filter circuitry that is configured to receive a system input signal and generate a first lowpass filtered output signal. The system also includes a first limiter circuitry that is configured to limit the first lowpass filtered output signal. Limiting the first low pass filtered output signal includes reducing an amplitude of the first lowpass filtered output signal below a first predetermined threshold value, thereby to generate a first limited output signal. A second lowpass filter circuitry is configured to receive the first limited output signal and to generate a second lowpass filtered output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/918/145, filed Oct. 20, 2015, which is incorporated herein byreference in its entirety.

BACKGROUND

This disclosure relates to signal processing. More particularly, thepresent disclosure related to a system and method for distortionlimiting.

In an audio system, when the amplitude of an audio signal exceeds themaximum voltage capability of the system the output signal stopsabruptly at the voltage limit causing the tops and bottoms of the audiowave form to be, in effect, cut-off, resulting in clipping. To a worseextent, in the frequency range that is below the resonant frequency of aloudspeaker, for example a woofer, substantial distortion could beproduced even when the speaker is driven by a voltage well below themaximum level. To address these problems, limiters are often used tolimit the output voltage to speakers at high output levels.

A conventional limiter is a single band configuration that applies alimiting gain to all frequencies equally. Since music is generally adynamic and broadband signal, such a configuration may cast unfavorableperformance limitations to an audio system. For example, one prominentdrawback with the conventional limiter is that there is often acompromise between bandwidth and maximum output sound level. Morespecifically, if the system engineer prefers louder sound for a speakerat maximum output, which is typically true, he/she would have to reducethe operation bandwidth of that speaker in order to meet the distortionperformance threshold, as illustrated by line 102 of FIG. 1A. Thebandwidth cut back is typically done by increasing the cut off frequencyof the high pass filter in the system equalizations, which willeffectively remove much of the bass content. As a result, a substantialamount of bass capability of the speaker cannot be utilized. On theother hand, if the system engineer prefers to retain the deeper bassfrequencies, he/she would have to tune the limiter in a very aggressiveway to lower the sound level at maximum output in order to meet thedistortion performance requirement, as illustrated by line 104 of FIG.1A.

SUMMARY

In one aspect, a system is provided for limiting distortion of an audiospeaker. The system includes a first lowpass filter circuitry that isconfigured to receive a system input signal and generate a first lowpassfiltered output signal. The system also includes a first limitercircuitry that is configured to limit the first lowpass filtered outputsignal. Limiting the first low pass filtered output signal includesreducing an amplitude of the first lowpass filtered output signal belowa first predetermined threshold value, thereby to generate a firstlimited output signal. A second lowpass filter circuitry is configuredto receive the first limited output signal and to generate a secondlowpass filtered output signal.

Implementations may include one of the following features, or anycombination thereof.

In some implementations, the system also includes a highpass filtercircuitry that is configured to receive a system input signal and togenerate a highpass filtered output signal. A second limiter circuitryis configured to limit the highpass filtered output signal. Limiting thehighpass filtered output signal includes reducing an amplitude of thehighpass filtered output signal below a second predetermined thresholdvalue, thereby to generate a second limited output signal. A firstcombiner circuitry is configured to receive and combine respectivesignals (e.g., the first and second limited output signals) from thefirst limiter circuitry and the second limiter circuitry to provide afirst combined output signal. The second lowpass filter circuitry isconfigured to filter the first combined output signal to generate thesecond lowpass filtered output signal. A third limiter circuitry isconfigured to limit the second lowpass filtered output signal. Limitingthe second lowpass filtered output signal includes reducing an amplitudeof the second lowpass filtered output signal below a third predeterminedthreshold value, thereby to provide a distortion limiting output signal.

In certain implementations, an allpass filter circuitry is configured toreceive a signal (e.g., the second limited signal) from the secondlimiter circuitry and to generate an allpass filtered output signal, anda second combiner circuitry is configured to combine the second lowpassfiltered output signal and the allpass filtered output signal to providea second combined output signal. Limiting the second lowpass filteredsignal includes limiting the second combined output signal.

In some examples, the system includes polarity circuitry arrangedbetween the allpass filter circuitry and the second combiner circuitry,and configured to adjust a polarity of the allpass filtered outputsignal before transmission to the second combiner circuitry.

In certain examples, the system includes a first equalization circuitrythat is configured to boost an amplitude of a first set of frequencycomponents in the system input signal, and a second equalizationcircuitry that is configured to reduce the amplitude of the first set offrequency components in the distortion limiting output signal.

In some cases, the system also includes a highpass filter circuitry thatis configured to receive a system input signal. The highpass filtercircuitry is configured to generate a highpass filtered output signaland to provide the highpass filtered output signal to the second limitercircuitry. The system may also include a second limiter circuitryconfigured to limit the highpass filtered output signal. Limiting thehighpass filtered output signal includes reducing an amplitude of thehighpass filtered output signal below a second predetermined thresholdvalue, thereby to generate a second limited output signal. A firstcombiner circuitry is configured to combine the second lowpass filteredoutput signal with the output from the second limiter circuitry toprovide a first combined output signal. A third limiter circuitry isconfigured to limit the first combined output signal. Limiting the firstcombined output signal includes reducing an amplitude of the firstcombined output signal below a third predetermined threshold value,thereby to provide a distortion limiting output signal.

In certain cases, the system can include a first equalization circuitrythat is configured to boost an amplitude of a first set of frequencycomponents in the system input signal, and a second equalizationcircuitry that is configured to reduce the amplitude of the first set offrequency components in the second filtered output signal.

Another aspect provides a method for limiting distortion of an audiospeaker. The method includes lowpass filtering a system input signal toprovide a first lowpass filtered output signal. The first lowpassfiltered output signal is limited to provide a first limited outputsignal. Limiting the first lowpass filtered output signal includesreducing an amplitude of the first lowpass filtered output signal belowa first predetermined threshold value. The method also includes lowpassfiltering the first limited output signal to provide a second lowpassfiltered output signal.

Implementations may include one of the above and/or below features, orany combination thereof.

In some implementations, the second lowpass filtered output signal istransduced to acoustic energy via an electro-acoustic transducer.

In certain implementations, the system input signal is highpass filteredto provide a highpass filtered output signal. The highpass filteredoutput signal is limited to provide a second limited output signal.Limiting the highpass filtered output signal includes reducing anamplitude of the highpass filtered output signal below a secondpredetermined threshold value. The first limited output signal and thesecond limited output signal are combined to provide a first combinedoutput signal. The step of lowpass filtering the first limited outputsignal includes lowpass filtering the first combined output signal toprovide the second lowpass filtered output signal. The second lowpassfiltered output signal is limited to provide a distortion limitingoutput signal. Limiting the second lowpass filtered output signalincludes reducing an amplitude of the second lowpass filtered outputsignal below a third predetermined threshold value.

Methods can include transducing the distortion limiting output signal toacoustic energy via an electro-acoustic transducer.

In some examples, the second limited output signal is allpass filteredto provide an allpass filtered output signal. The second lowpassfiltered output signal and the allpass filtered output signal arecombined to provide a second combined output signal. The step oflimiting the second lowpass filtered output signal may include limitingthe second combined output signal.

In certain examples, a polarity of the allpass filtered output signal isadjusted (e.g., reversed) to provide a polarity adjusted output signal.The step of combining the second lowpass filtered output signal and theallpass filtered output signal may include combining the second lowpassfiltered output signal and the polarity adjusted output signal toprovide the second combined output signal.

In some cases, the system input signal is equalized to boost anamplitude of a first set of frequency components in the system inputsignal, and the distortion limiting output signal is equalized to reducethe amplitude of the first set of frequency components in the distortionlimiting output signal.

In certain cases, the system input signal is highpass filtered toprovide a highpass filtered output signal. The highpass filtered outputsignal is then limited to provide a second limited output signal.Limiting the highpass filtered output signal includes reducing anamplitude of the highpass filtered output signal below a secondpredetermined threshold value. The second limited output signal and thesecond lowpass filtered output signal are combined to provide a firstcombined output signal. The first combined output signal is limited toprovide a distortion limiting output signal. Limiting the first combinedoutput signal includes reducing an amplitude of the first combinedoutput signal below a third predetermined threshold value.

In yet another aspect, a system includes a first limiter circuitry, anda first lowpass filter circuitry that provides a first lowpass filteredoutput signal to the first limiter circuitry. The system also includes asecond limiter circuitry, and a highpass filter circuitry that providesa highpass filtered output signal to the second limiter circuitry. Afirst combiner circuitry is configured to combine respective signalsfrom the first and second limiter circuitry and provides a firstcombined output signal. A second lowpass filter circuitry receives thefirst combined output signal and generates a second lowpass filteredoutput signal. An allpass filter circuitry receives a signal from thesecond limiter circuitry and generates an allpass filtered outputsignal. A second combiner circuitry combines the second lowpass filteredoutput signal and the allpass filtered output signal to provide a secondcombined output signal. A third limiter circuitry receives the secondcombined output signal and provides a distortion limiting output signal.

Implementations may include one of the above and/or below features, orany combination thereof.

In some implementations, the system also includes polarity circuitrythat is arranged between the allpass filter circuitry and the secondcombiner circuitry. The polarity circuitry is configured to adjust(e.g., reverse) a polarity of the second lowpass filtered output signalbefore transmission to the second combiner circuitry.

In certain implementations, the first limiter circuitry includes a firstlimiter, the second limiter circuitry includes a second limiter, and thefirst and second limiters are tuned independently with each having itsown threshold and time constants.

In some examples, the allpass filter circuitry includes a 1st order allpass filter or a 2nd order allpass filter.

Another aspect features a method for limiting distortion of an audiospeaker. The method includes: receiving a system input signal at a firstlowpass filter circuitry; generating a first lowpass filtered outputsignal at the first lowpass filter circuitry; receiving the firstlowpass filtered output signal at a first limiter circuitry; limitingthe first lowpass filtered output signal via the first limiter circuitryto generate a first limited output signal. Limiting the first lowpassfiltered output signal includes reducing an amplitude of the firstlowpass filtered output signal below a first predetermined thresholdvalue. The system input signal is received at a highpass filtercircuitry and a highpass filtered output signal is generated at thehighpass filter circuitry. The highpass filtered output signal islimited at the second limiter circuitry to generate a second limitedoutput signal. Limiting the highpass filtered output signal includesreducing an amplitude of the highpass filtered output signal below asecond predetermined threshold value. The first limited output signaland second limited output signal are combined to generate a firstcombined output signal. The first combined output signal is received ata second lowpass filter circuitry, and a second lowpass filtered outputsignal is generated at the second lowpass filter circuitry. The secondlimited output signal is received at an allpass filter circuitry, and anallpass filtered output signal is generated at the allpass filtercircuitry. The second lowpass filtered output signal and the allpassfiltered output signal are combined to generate a second combined outputsignal. The second combined output signal is limited via the thirdlimiter circuitry to generate a distortion limiting output signal.Limiting the second combined output signal includes reducing anamplitude of the second combined output signal below a thirdpredetermined threshold value.

Implementations may include one of the above and/or below features, orany combination thereof.

In some implementations, the method also includes receiving thedistortion limiting output signal at an electro-acoustic transducer; andtransducing the distortion limiting output signal to acoustic energy viathe electro-acoustic transducer.

In certain implementations, the method includes reversing a polarity ofthe allpass filtered output signal using polarity circuitry providedbetween the allpass filter circuitry and the second combiner circuitry.

In some examples, at least one of the first lowpass filter circuitry,the second lowpass filter circuitry, and the highpass filter circuitryis configured as a 5th order Butterworth filter.

In certain examples, the allpass filter circuitry includes a 1st orderallpass filter or a 2nd order allpass filter.

Other aspects, features, and advantages are in the description,drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is diagram illustrating a bandwidth and sound level tradeoff foran audio system with a conventional limiter for limiting distortion inthe audio system.

FIG. 1B is diagram illustrating the performance of the distortionlimiting system of the present disclosure as compared to theconventional limiters.

FIG. 2A depicts a block diagram showing a first embodiment of adistortion limiting system for reducing distortion in the output of anaudio system.

FIG. 2B depicts a block diagram showing a second embodiment of adistortion limiting system for reducing distortion in the output of anaudio system.

FIG. 2C depicts a block diagram showing a third embodiment of adistortion limiting system for reducing distortion in the output of anaudio system.

FIG. 2D depicts a block diagram showing a fourth embodiment of adistortion limiting system for reducing distortion in the output of anaudio system

FIG. 3 depicts a flowchart showing operations associated with a methodconsistent with the embodiment of FIG. 2D.

DETAILED DESCRIPTION

This disclosure is based on a distortion limiting system that isproposed to help resolve the compromise between bandwidth and maximumsound level (max SPL). The idea is to substantially extend the bassfrequency range at normal volume levels and maintain the sound level atmaximum output as compared to conventional limiters, as illustrated byline 106 of FIG. 1B. This could lead to significant performanceimprovement, especially for audio systems which do not include adedicated bass box.

As used in any embodiment described herein, the term “circuitry” maycomprise, for example, singly or in any combination, digital circuitry,analog circuitry, hardwired circuitry, programmable circuitry, statemachine circuitry, and/or firmware that stores instructions executed byprogrammable circuitry. It should be understood at the outset that anyof the operations and/or operative components described in anyembodiment herein may be implemented in software, firmware, hardwiredcircuitry and/or any combination thereof.

As used in any embodiment described herein, the terms “coupled” and“operatively connected”, as used herein, may refer to one or morecomponents that are configured to communicate a signal. The coupledcomponents may be physically connected by electrically conductive wireor by optically transmissive fiber, or may be communicatingly coupled bya wireless technique such as infrared or radio frequency (RF), or othersignal communication techniques. Similarly, the term “signal line” asused herein, may refer to any transmissive path, including electricallyconductive wire, optically transmissive fiber, a wireless communicationpath, or other type of signal transmission path, for transmitting analogor digitally encoded signals.

As used herein “limiting” is distinguished from “clipping.” Clippingabruptly stops an audio signal from going over a maximum voltage limit,and so the tops and bottoms of the audio wave form are, in effect,cut-off. This is something that inevitably produces strong harmonicdistortion. The result is a system that is entirely linear up to theclipping level, and then horribly non-linear, so signals slightly lowerthan the clipping level are passed unaffected, while those at, or higherthan, the clipping level are mangled, resulting in the addition ofcomplex additional harmonics that were not present in the original.

Limiting is an altogether much more controlled business, where a loudsignal is briefly attenuated, specifically to avoid clipping. The resultis a system that is linear up to the limiting level, above which thewaveform is reduced in amplitude but more or less retains its originalshape, and thus remains harmonically intact.

Referring to FIG. 2A, an embodiment of the present disclosure depictingdistortion limiting system 200 is provided. The system 200 of FIG. 2A isshown as a block diagram. There may or may not be physical elementscorresponding to each of the elements of FIG. 2A. For example, a systeminput signal 202 may be received at one or more input terminals (notshown), which may be implemented as a single physical input terminalreceiving a stream of signals. Elements such as the lowpass filteringcircuitries, highpass filtering circuitries, or others, may beimplemented by a digital signal processor (DSP) operating on digitallyencoded data. Additionally and or alternatively, other circuitarrangements may produce substantially the same result as thearrangement of FIG. 2A. More than one of the blocks may be representedby a single element, or blocks may be consolidated.

In some embodiments, distortion limiting system 200 may be configured toreceive a system input signal 202, which may be provided by an amplifier(not shown) or alternative device through one or more signal lines. Theinput signal 202 may be provided to first lowpass filter circuitry 204.The first lowpass filter circuitry 204 may be operatively connected tofirst limiter circuitry 210, which may be configured to attenuate theamplitude of the signal entering the first limiter circuitry 210 if itexceeds a predefined threshold. The output of the first limitercircuitry 210 passed through second lowpass filter circuitry 216 toprovide a distortion limiting output signal 226, which can be transducedto acoustic energy via an electro-acoustic transducer.

In some embodiments, the system 200 may include first and secondequalization circuitry 230, 232. The first equalization circuitry 230 isconfigured to perform an equalization operation on the input signalbefore it is provided to the first lowpass filter circuitry 204 and/orthe highpass filter circuitry 206. In that regard, the firstequalization circuitry 230 may be configured to boost (i.e., increasethe magnitude of) certain frequencies in the input signal 202 to causethe limiter circuitry downstream to limit those frequencies at a lowersignal level than they ordinarily would be limited. The secondequalization circuitry 232 is arranged to equalize the output signal226. The second equalization circuitry 232 is complimentary to the firstequalization circuitry 230 and cuts (i.e., reduces the magnitude of) thevery same frequencies that were boosted upstream by the firstequalization circuitry 230. This equalization circuitry enables thesystem to effectively have different limiting thresholds for differentfrequency ranges.

In some embodiments, the high frequency content of the input signal 202may also be limited to attenuate the amplitude of the high frequencycontent. This can be done separately from the limiting of the lowfrequency content. The limited high frequency content and the limitedlow frequency content can then be combined to provide an output signalthat includes the full frequency band of the input signal. For example,in the embodiment illustrated in FIG. 2B, the system 200 has a highfrequency path that includes highpass filter circuitry 206. The highpassfilter circuitry 206 may receive system input signal 202 through one ormore signal lines. Optionally, the highpass filter circuitry 206 may beoperatively connected to a second limiter circuitry 214, which may beconfigured to attenuate the amplitude of the highpass filtered signalentering the second limiter circuitry 214 if it exceeds a predefinedthreshold.

In some cases, first combiner circuitry 212 may be operatively connectedto the first limiter circuitry 210, the highpass filter circuitry 206(or the second limiter circuitry 214 when utilized), and second lowpassfilter circuitry 216. The second lowpass filter circuitry 216 may beoperatively connected to third (master) limiter circuitry 224. The firstcombiner circuitry 212 may receive an input from each of the firstlimiter circuitry 210 and the second limiter circuitry 214—or directlyform the highpass filter circuitry 206 when the second limiter circuitryis not utilized. The output of first combiner circuitry 212 is providedto the second lowpass filter circuitry 216. The output of second lowpassfilter circuitry 216 is provided to the third (master) limiter circuitry224 to generate the distortion limiting output signal 226.

Alternatively, as illustrated in FIG. 2C, the first combiner circuitry212 may be configured to the receive an input from the second lowpassfilter circuitry 216 and the second limiter circuitry 214, such that thehigh frequency content and the low frequency content is combined afterthe second lowpass filtering operation is performed. The output of thefirst combiner circuitry 212 may then be provided to the third limitercircuitry 224 to generate the distortion limiting output signal 226.

In some cases, the second limiter circuitry 214 may be operativelyconnected to allpass filter circuitry 220, as shown in FIG. 2D. Theallpass filter circuitry 220 may be operatively connected to secondcombiner circuitry 218, either directly or via polarity circuitry 222.The second combiner circuitry 218 may receive an input from each of thesecond lowpass filter 216 and the polarity circuitry 222. The output ofsecond combiner circuitry 218 is provided to third limiter circuitry224.

In the embodiment illustrated in FIG. 2D, the input signal 202 is bandsplit into two frequency ranges one for the bass region, via the firstlowpass filter 204, and the other one for all higher frequencies, viathe highpass filter 206. Each band has its own limiter (i.e., first andsecond limiter circuitry 210, 214) that works independently from theother. The output of the two frequency bands are summed back up to forma full band signal which is lowpassed again, via the second lowpassfilter circuitry 216, to remove the harmonic distortions from thelimiting processes. The highpass filtered signal is phase adjusted bythe allpass filter circuitry 220 and then summed up again with theoutput of the second lowpass filter circuitry 216. Finally, the summedsignal goes through a master limiter, provided by third limitercircuitry 224, for further protections. In that regard, the output ofthe second combiner circuitry 218 can be clipped and cause newdistortion. The third limiter circuitry 224 helps to prevent suchevents.

One or more of the first lowpass filter circuitry 204, the secondlowpass filter circuitry 216, and the highpass filter circuitry 206 maybe configured as a Butterworth filter. However, numerous other filtertypes may be used without departing from the scope of this disclosure.Some of these may include, but are not limited to, Chebyshev, Elliptic,Bessel, Gaussian, Legendre, Linkwitz-Riley, etc.

In some embodiments, one or more of the first lowpass filter circuitry204, the second lowpass filter circuitry 216, the highpass filtercircuitry 206, and the allpass filter circuitry 220 may have the same orsubstantially similar corner frequencies. For the purposes of thisdiscussion, the corner frequency may refer to the frequency at whichattenuation begins to increase sharply; or for the allpass filter, itscorner frequency may be where its phase has traversed half of its fullphase change (for 1st order, that phase is 90 degrees; for 2nd order,that phase is 180 degrees).

Any or all of the filtering circuitries described herein, such as thefirst lowpass filtering circuitry 204 and the highpass filteringcircuitry 206, may be configured as odd ordered filters, which mayinclude, but are not limited to, 3rd or 5th order filters. Otherconfigurations are also within the scope of the present disclosure.

The two limiters in the lowpass and highpass branches (i.e., the firstlimiter circuitry 210 and the second limiter circuitry 214,respectively) are tuned independently each with its own threshold andtime constants. In some embodiments, the tuning parameters of the thirdlimiter circuitry 224 can be set to be the same as in the second limitercircuitry 214.

Referring to FIG. 3 and in operation, a method 300 for limiting thedistortion of an audio speaker may include receiving 302 the systeminput signal 202 at the first lowpass filter circuitry 204. In somecases, the system input signal 202 may be an equalized 301, prior toreceipt, in order to boost a set of frequency components in the systeminput signal 202, thereby to cause a downstream limiting operation tolimit those frequencies at a lower signal level than they ordinarilywould be limited The system input signal 202 may be transmitted alongone or more signal lines from another device, e.g. an amplifier, to thefirst lowpass filter circuitry 204. The first lowpass filter circuitry204 may receive the system input signal 202 and may generate 304 a firstlowpass filtered output signal. The first lowpass filtered output signalmay be transmitted along one or more signal lines to the first limitercircuitry 210. The first limiter circuitry 210 may limit 306, i.e.,reduce the amplitude, of the first lowpass filtered output signal tolower it below a predetermined threshold value to provide a firstlimited output signal. Additionally or alternatively, the first lowpassfiltered output signal may be transmitted directly to the first combinercircuitry 212, such as when the first limiter circuitry 210 determinesthat no limiting is necessary.

The method 300 may further include receiving 308 the system input signal202 at the highpass filter circuitry 206. The highpass filter circuitry206 may generate 310 a highpass filtered output signal and may transmitthe highpass filtered output signal to second limiter circuitry 214. Thesecond limiter circuitry 214 may limit 312, i.e., reduce the amplitude,of the highpass filtered output signal to lower it below a predeterminedthreshold value to provide a second limited output signal. Additionallyor alternatively, the highpass filtered output signal may be transmitteddirectly to first combiner circuitry 212, such as when the secondlimiter circuitry 214 determines that no limiting is necessary.

The first combiner circuitry 212 may receive 314, 316 the outputs fromthe first and second limiter circuitry 210, 214, which may be combined318 to generate a first combined output signal. Combining 318 theoutputs from the first and second limiters 210, 214 may include addingor subtracting the two signals.

The first combined output signal may be transmitted by first combinercircuitry 212, along one or more signal lines, to the second lowpassfilter circuitry 216. The second lowpass filter circuitry 216 mayreceive 320 the first combined output signal and may generate 322 asecond lowpass filtered output signal. The second lowpass filtercircuitry 216 may provide the second lowpass filtered output signal tothe second combiner circuitry 218.

Along the highpass branch, method 300 may further include receiving 324the output of the second limiter circuitry at the allpass filtercircuitry 220. As discussed above, the allpass filter circuitry 220 maybe configured in a number of different arrangements, e.g. as a 1st orderfilter, 2nd order filter, etc. The allpass filter circuitry 220 mayadjust the phase of the output of the second limiter circuitry andthereby generate 326 an allpass filtered output signal. The allpassfiltered output signal may be transmitted from the allpass filtercircuitry 220 to the polarity circuitry 222. The polarity circuitry 222may reverse the polarity 328 of the allpass filtered output signalbefore transmission to the second combiner circuitry 218. The secondcombiner circuitry 218 may receive and combine 330 the second lowpassfiltered output signal and the allpass filtered output signal in orderto generate a second combined output signal.

The second combiner circuitry 218 may transmit the second filteredoutput signal to the third limiter circuitry 224. The third limitercircuitry 224 may limit 332, i.e., reduce the amplitude, of the secondcombined output signal to lower it below a predetermined threshold valuein order to generate a distortion limiting output signal 226, which canbe transduced via an electro-acoustic transducer. In some cases, thedistortion limiting output signal 226 may be equalized 334 before it istransduced in order to reduce the amplitude of a set of frequencycomponents in the distortion limiting output signal 226, such as whenthe same set of frequency components is boosted in the system inputsignal 202. Such complimentary equalization can enable differentlimiting thresholds for different frequency ranges.

A number of embodiments have been described. Nevertheless, it will beunderstood that additional modifications may be made without departingfrom the scope of the inventive concepts described herein, and,accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A system for limiting distortion of an audiospeaker, the system comprising: a first lowpass filter circuitryconfigured to receive a system input signal and generate a first lowpassfiltered output signal; a first limiter circuitry configured to limitthe first lowpass filtered output signal, wherein limiting the first lowpass filtered output signal comprises reducing an amplitude of the firstlowpass filtered output signal below a first predetermined thresholdvalue, thereby to generate a first limited output signal; a secondlowpass filter circuitry configured to receive the first limited outputsignal and to generate a second lowpass filtered output signal.
 2. Thesystem of claim 1, further comprising: a highpass filter circuitryconfigured to receive a system input signal, the highpass filtercircuitry configured to generate a highpass filtered output signal; asecond limiter circuitry configured to limit the highpass filteredoutput signal, wherein limiting the highpass filtered output signalcomprises reducing an amplitude of the highpass filtered output signalbelow a second predetermined threshold value, thereby to generate asecond limited output signal; a first combiner circuitry configured toreceive and combine respective signals from the first limiter circuitryand the second limiter circuitry to provide a first combined outputsignal, wherein the second lowpass filter circuitry is configured tofilter the first combined output signal to generate the second lowpassfiltered output signal; and a third limiter circuitry configured tolimit the second lowpass filtered output signal, wherein limiting thesecond lowpass filtered output signal comprises reducing an amplitude ofthe second lowpass filtered output signal below a third predeterminedthreshold value, thereby to provide a distortion limiting output signal.3. The system of claim 2, further comprising: an allpass filtercircuitry configured to receive a signal from the second limitercircuitry and to generate an allpass filtered output signal; a secondcombiner circuitry configured to combine the second lowpass filteredoutput signal and the allpass filtered output signal to provide a secondcombined output signal, wherein limiting the second lowpass filteredsignal includes comprises the second combined output signal.
 4. Thesystem of claim 3, further comprising polarity circuitry arrangedbetween the allpass filter circuitry and the second combiner circuitry,and configured to adjust a polarity of the allpass filtered outputsignal before transmission to the second combiner circuitry.
 5. Thesystem of claim 2, further comprising a first equalization circuitryconfigured to boost an amplitude of a first set of frequency componentsin the system input signal; and a second equalization circuitryconfigured to reduce the amplitude of the first set of frequencycomponents in the distortion limiting output signal.
 6. The system ofclaim 1, further comprising: a highpass filter circuitry configured toreceive a system input signal, the highpass filter circuitry configuredto generate a highpass filtered output signal and to provide thehighpass filtered output signal to the second limiter circuitry; asecond limiter circuitry configured to limit the highpass filteredoutput signal, wherein limiting the highpass filtered output signalcomprises reducing an amplitude of the highpass filtered output signalbelow a second predetermined threshold value, thereby to generate asecond limited output signal; a first combiner circuitry configured tocombine the second lowpass filtered output signal with the output fromthe second limiter circuitry to provide a first combined output signal;and a third limiter circuitry configured to limit the first combinedoutput signal, wherein limiting the first combined output signalcomprises reducing an amplitude of the first combined output signalbelow a third predetermined threshold value, thereby to provide adistortion limiting output signal.
 7. The system of claim 6, furthercomprising a first equalization circuitry configured to boost anamplitude of a first set of frequency components in the system inputsignal; and a second equalization circuitry configured to reduce theamplitude of the first set of frequency components in the secondfiltered output signal.
 8. The system of claim 1, further comprising afirst equalization circuitry configured to boost an amplitude of a firstset of frequency components in the system input signal; and a secondequalization circuitry configured to reduce the amplitude of the firstset of frequency components in the second filtered output signal.
 9. Thesystem of claim 1, further comprising: a highpass filter circuitryconfigured to receive a system input signal, the highpass filtercircuitry configured to generate a highpass filtered output signal; afirst combiner circuitry configured to receive and combine respectivesignals from the first limiter circuitry and the highpass filtercircuitry to provide a first combined output signal, wherein the secondlowpass filter circuitry is configured to filter the first combinedoutput signal to generate the second lowpass filtered output signal; anda master limiter circuitry configured to limit the second lowpassfiltered output signal, wherein limiting the second lowpass filteredoutput signal comprises reducing an amplitude of the second lowpassfiltered output signal below a master threshold value, thereby toprovide a distortion limiting output signal.
 10. The system of claim 9,further comprising a first equalization circuitry configured to boost anamplitude of a first set of frequency components in the system inputsignal; and a second equalization circuitry configured to reduce theamplitude of the first set of frequency components in the distortionlimiting output signal.
 11. A method for limiting distortion of an audiospeaker, the method comprising: lowpass filtering a system input signalto provide a first lowpass filtered output signal; limiting the firstlowpass filtered output signal to provide a first limited output signal,wherein limiting the first lowpass filtered output signal comprisesreducing an amplitude of the first lowpass filtered output signal belowa first predetermined threshold value; and lowpass filtering the firstlimited output signal to provide a second lowpass filtered outputsignal.
 12. The method of claim 11, further comprising: transducing thesecond lowpass filtered output signal to acoustic energy via anelectro-acoustic transducer.
 13. The method of claim 11, furthercomprising highpass filtering the system input signal to provide ahighpass filtered output signal; limiting the highpass filtered outputsignal to provide a second limited output signal, wherein limiting thehighpass filtered output signal comprises reducing an amplitude of thehighpass filtered output signal below a second predetermined thresholdvalue; combining the first limited output signal and the second limitedoutput signal to provide a first combined output signal; wherein lowpassfiltering the first limited output signal comprises lowpass filteringthe first combined output signal to provide the second lowpass filteredoutput signal; and limiting the second lowpass filtered output signal toprovide a distortion limiting output signal, wherein limiting the secondlowpass filtered output signal comprises reducing an amplitude of thesecond lowpass filtered output signal below a third predeterminedthreshold value.
 14. The method of claim 13, further comprising:transducing the distortion limiting output signal to acoustic energy viaan electro-acoustic transducer.
 15. The method of claim 13, furthercomprising: allpass filtering the second limited output signal toprovide an allpass filtered output signal; and combining the secondlowpass filtered output signal and the allpass filtered output signal toprovide a second combined output signal, wherein limiting the secondlowpass filtered output signal comprises limiting the second combinedoutput signal.
 16. The method of claim 15, further comprising reversinga polarity of the allpass filtered output signal to provide a polarityadjusted output signal, wherein combining the second lowpass filteredoutput signal and the allpass filtered output signal comprises combiningthe second lowpass filtered output signal and the polarity adjustedoutput signal to provide the second combined output signal.
 17. Themethod of claim 15, further comprising: equalizing the system inputsignal to boost an amplitude of a first set of frequency components inthe system input signal; and equalizing the distortion limiting outputsignal to reduce the amplitude of the first set of frequency componentsin the distortion limiting output signal.
 18. The method of claim 11,further comprising: highpass filtering the system input signal toprovide a highpass filtered output signal; limiting the highpassfiltered output signal to provide a second limited output signal,wherein limiting the highpass filtered output signal comprises reducingan amplitude of the highpass filtered output signal below a secondpredetermined threshold value; combining the second limited outputsignal and the second lowpass filtered output signal to provide a firstcombined output signal; limiting the first combined output signal toprovide a distortion limiting output signal, wherein limiting the firstcombined output signal comprises reducing an amplitude of the firstcombined output signal below a third predetermined threshold value. 19.A system for limiting distortion of an audio speaker, the systemcomprising: a first limiter circuitry; a first lowpass filter circuitryconfigured to receive a system input signal, the first lowpass filtercircuitry configured to generate a first lowpass filtered output signaland to provide the first lowpass filtered output signal to the firstlimiter circuitry; a second limiter circuitry; a highpass filtercircuitry configured to receive a system input signal, the highpassfilter circuitry configured to generate a highpass filtered outputsignal and to provide the highpass filtered output signal to the secondlimiter circuitry; a first combiner circuitry configured to receive andcombine respective signals from the first limiter circuitry and thesecond limiter circuitry to provide a first combined output signal; asecond lowpass filter circuitry configured to receive the first combinedoutput signal and to generate a second lowpass filtered output signal;an allpass filter circuitry configured to receive a signal from thesecond limiter circuitry and to generate an allpass filtered outputsignal; a second combiner circuitry configured to receive and combinethe second lowpass filtered output signal and the allpass filteredoutput signal to provide a second combined output signal; and a thirdlimiter circuitry configured to receive the second combined outputsignal and to provide a distortion limiting output signal.
 20. A methodfor limiting distortion of an audio speaker, the method comprising:receiving a system input signal at a first lowpass filter circuitry;generating a first lowpass filtered output signal at the first lowpassfilter circuitry; receiving the first lowpass filtered output signal ata first limiter circuitry; limiting the first lowpass filtered outputsignal via the first limiter circuitry to generate a first limitedoutput signal, wherein limiting the first lowpass filtered output signalcomprises reducing an amplitude of the first lowpass filtered outputsignal below a first predetermined threshold value; receiving a systeminput signal at a highpass filter circuitry; generating a highpassfiltered output signal at the highpass filter circuitry; receiving thehighpass filtered output signal at a second limiter circuitry; limitingthe highpass filtered output signal via the second limiter circuitry togenerate a second limited output signal, wherein limiting the highpassfiltered output signal comprises reducing an amplitude of the highpassfiltered output signal below a second predetermined threshold value;receiving the first limited output signal and the second limited outputsignal at a first combiner circuitry; combining, at the first combinercircuitry, the first limited output signal and second limited outputsignal to generate a first combined output signal; receiving at a secondlowpass filter circuitry the first combined output signal; generating asecond lowpass filtered output signal at the second lowpass filtercircuitry; receiving at an allpass filter circuitry the second limitedoutput signal; generating an allpass filtered output signal at theallpass filter circuitry; receiving the second lowpass filtered outputsignal and the allpass filtered output signal at a second combinercircuitry; combining, at the second combiner circuitry, the secondlowpass filtered output signal and the allpass filtered output signal togenerate a second combined output signal; receiving the second combinedoutput signal at a third limiter circuitry; limiting the second combinedoutput signal via the third limiter circuitry to generate a distortionlimiting output signal, wherein limiting the second combined outputsignal comprises reducing an amplitude of the second combined outputsignal below a third predetermined threshold value.